As the reaction systems of a Fischer-Tropsch synthesis reaction (hereinafter called FT synthesis reaction) that generates a hydrocarbon compound and water by catalytic reaction from a synthesis gas which is mainly composed of hydrogen and carbon monoxide, a bubble column type slurry bed FT synthesis reaction system that carries out an FT synthesis reaction by introducing a synthesis gas into a slurry in which solid catalyst particles are suspended in a liquid hydrocarbon is available (for example, refer to Patent Document 1 and 2 as mentioned below). Further, a hydrocarbon compound synthesized by the FT synthesis reaction is mainly utilized as a raw material for liquid fuel products such as naphtha (rough gasoline), kerosene and gas oil. Further, a hydrocarbon compound synthesized by the FT synthesis reaction is mainly utilized as a raw material for liquid fuel products such as naphtha (rough gasoline), kerosene and gas oil.
In the bubble column type slurry bed FT synthesis reaction system, a synthesis gas introduced into the slurry ascends through the slurry as bubbles. In order to control the reaction state between catalyst particles and the synthesis gas in a state (operating state) where the synthesis gas is stably supplied into the slurry, it is necessary to know the concentration of the catalyst particles included in the slurry. However, since the amount of liquid hydrocarbons included in the slurry increases by an FT synthesis reaction, the concentration of the catalyst particles will change with this increase. Accordingly, when the reaction state is controlled, it is necessary to detect the liquid level position of the slurry serving as an index of the amount of the liquid hydrocarbons.
In addition, as a conventional liquid level detecting method, there is a method utilizing a float on the liquid level, for example, as disclosed in Patent Document 3, or a so-called capacitive method of making water or slurry including solid function as a component of a capacitor to thereby detect the liquid level (interface of gas and liquid) of water or slurry, for example, as disclosed in Patent Document 4. Further, as the conventional liquid level detecting method, there is a method of measuring the pressure of liquid accommodated in a vessel, the pressure of gas above the liquid level within the vessel, and the temperature of the liquid, and obtaining a liquid level position using the difference (differential pressure) between the two measured pressures, the distance between the two pressure detection positions, the specific gravity of the liquid calculated on the basis of the measured pressures and measured temperatures, etc., for example, as disclosed in Patent Document 5.    PATENT DOCUMENT 1: US Patent Application, First Publication No. 2003/0018089    PATENT DOCUMENT 2: US Patent Application, First Publication No. 2007/0014703    PATENT DOCUMENT 3: US Patent Application, First Publication No. 2004/0021080    PATENT DOCUMENT 4: U.S. Pat. No. 4,888,989    PATENT DOCUMENT 5: US Patent Application, First Publication No. 2006/0070438